Carbon nanotubes have very special properties, such as low density, high strength, high toughness, high flexibility, high surface area, high surface curvature, high thermal conductivity, and excellent electric conductivity, etc. That is why carbon nanotubes have attracted many researchers to study on the possible applications of the carbon nanotubes which include: composite material, microelectronic components, flat displays, radio communication, fuel cells, and lithium cells, etc. Carbon nanotube field emission displays (CNT-FED) are novel flat displays that have a great potential. Usually, a process for producing a large CNT-FED comprises: mixing carbon nanotubes with a conductive paste; coating the paste mixture on the surface of a conductive glass substrate by a screen printing technique, or the like; sintering the composite at 450-550° C. to remove the polymeric material in the paste mixture, thereby forming an electron emissive film having a good electrical conductivity. Such a CNT-FED production process requires several steps and uses a technique that is somehow cumbersome. Furthermore, the carbon nanotubes are difficult to be uniformly distributed in said conductive paste.
At present, the processes for producing nanotubes for use in the CNT-FED include: arc discharge, laser vaporization, and thermal CVD, etc. The carbon nanotube products prepared by the arc discharge process and the laser vaporization process not only are difficult to be controlled as to the length and the diameter thereof, but they are produced in a rather low yield. Furthermore, those processes will generate a large amount of amorphous carbon, so that further purification treatments are required. Moreover, these processes require a fabrication temperature exceeding 1000° C. such that carbon nanotubes can not be produced directly on a glass substrate. Therefore, it is widely recognized that a thermal CVD has the best possibility for producing carbon nanotubes at a lower temperature.
In the past, a process for producing carbon nanotubes by a thermal CVD uses an active metal catalyst deposited on a porous support such as silica, zeolite, alumina or magnesium oxide. The main reason in selecting the abovementioned supports is that such supports are stable inert oxides, and they will not react with the active metal catalyst inadvertently during a heating process, so that the active metal catalyst can catalyze a synthesis reaction of the carbon nanotubes as desired. The active metal mainly comprises: Fe, Co or Ni, and a minor quantity of other metals, such as Cu, Mo, Mn, Zn or Pt, etc., for adjusting the reaction activities. The reaction conditions of using an active metal catalyst, which is deposited on a support, to catalyze a carbon accumulation reaction for forming carbon nanotubes include: introducing an inert gas (He, Ar, or N2), hydrogen and a carbon source gas into a reactor at a reaction temperature of 650-1000° C. and a pressure of 1-2 atm for a reaction time of 1-120 min. The carbon source used includes: a hydrocarbon or carbon monoxide (CO). Upon completion of the reactions, the support needs to be removed by acid washing in order to obtain purer carbon nanotubes for use in a CNT-FED or other applications.
In the current CNT-FED fabrication process, the abovementioned cumbersome steps are needed for adhering carbon nanotubes to the surface of the substrate. As a result, the distribution and the orientation of the carbon nanotubes on the surface of the substrate are influenced by many process factors, such as the purity and specification of the carbon nanotubes, the amount of addition and the dispersion of the carbon nanotubes in the paste/nanotubes mixing step, and the technique of the screen printing, etc. These factors inevitably reduce the yield of the CNT-FED, and thus increase the production cost thereof. However, most of the above problems will vanish if the carbon nanotubes can be grown directly on the surface of the substrate, thereby greatly improving the CNT-FED production process. Furthermore, the synthesis of carbon nanotubes will become a module in the CNT-FED production process. The whole production steps can be systematically monitored in one process, thereby increasing the yield of the CNT-FED.
Generally speaking, the strain temperature of the calcined temperature resistant glass can reach up to 650° C., while the strain temperature of the sodium glass is about 550° C. or lower. Therefore, if the thermal CVD is used to directly grow carbon nanotubes on the surface of the glass substrate, the thermal CVD temperature can not exceed the strain temperature of the glass substrate, i.e. preferably lower than 650° C. However, the thermal CVD temperature cannot be too low, since the catalytic activity of the thermal CVD catalyst will be reduced and become insufficient for use in the synthesis of the carbon nanotubes. Therefore, it is necessary to develop a high catalytic activity catalyst system which can be used in synthesizing the carbon nanotubes at a temperature lower than 650° C.
European Patent Application No. 1061041 A1 discloses a low temperature CVD device and a method for synthesizing carbon nanotubes using such a device. The method comprises dividing a reaction pipe in the device into a space adjacent to the gas input part, a first zone for pyrolyzing the input gases, and a space adjacent to the gas discharge part, a second zone for synthesizing carbon nanotubes by using the resulting pyrolyzed gases; and maintaining the temperatures of the two zones so that the temperature of the second zone is lower than the temperature of the first zone. Two different catalyst substrates are used in the synthesizing zone of carbon nanotubes, wherein one substrate has an assist catalyst such as Pd, Cr and Pt, etc., which is mainly used to accelerate the pyrolysis of acetylene; the other substrate is deposited with a catalyst layer containing Fe, Co, Ni or an alloy thereof, which is a catalyst for synthesizing the carbon nanotubes. Said other catalyst substrate having a catalyst membrane containing Fe, Co, Ni or an alloy thereof is corroded by an etching gas to form nano-grade catalytic particles. The abovementioned device is used to pyrolyze a carbon source gas in the first zone by the assist catalyst. Then, in the second zone, the carbon source gas, which has been decomposed, is used to grow perpendicularly aligned carbon nanotubes on each isolated nano-grade catalytic particle on the substrate by the thermal CVD at a temperature equal to or lower than the strain temperature of the substrate. This prior art technique, in addition to using a low temperature reaction zone of 450-650° C., still needs to pyrolyze the carbon source gas (first zone) at a high temperature of 700-1000° C., and is not a pure low temperature process. This prior art technique also needs to use a special CVD reactor. Furthermore, in this prior art technique, it is necessary to form two types of metal catalyst layers on two substrates, and the two substrates are mounted in the thermal CVD such that the two metal layers are facing each other at a clearance. Obviously, this prior art technique is complex, costly, and difficult to be implemented.
European Patent Application No. 1061043 A1 discloses a method for synthesizing carbon nanotubes at a low temperature by using a metal catalyst layer, which comprises: forming a metal catalyst layer on a substrate, wherein said metal catalyst layer is etched to form isolated nano-grade catalytic metal particles; and growing perpendicularly aligned carbon nanotubes on each isolated nano-grade catalytic particle on the substrate by a thermal CVD by passing a pyrolyzed carbon source gas at a temperature equal to or lower than the strain temperature of the substrate. Said pyrolyzed carbon source gas is formed by using a carbon-source-gas decomposing metal catalyst layer. In this prior art technique, it is necessary to form two different metal catalyst layers on two substrates, and then the two substrates are mounted in a thermal CVD reactor such that the metal layers are facing each other at a clearance. Obviously, this prior art technique is an improvement to the process disclosed in the above-mentioned EP1061041 A1. The major improvement comprises modifying a two-staged heating system into a one-staged heating system. However, this prior art technique has no conspicuous improvement over the catalyst system, which still requires the use of two different catalyst systems on two substrates.